There are many existing perimeter security systems that use optical fiber as the sensing medium. Initially these systems were based on using the fiber as a wave-guide for a light signal and then detecting the light on the opposite end of the fiber. A loss of light would trigger an alarm. For example, U.S. Pat. No. 4,399,430 to Kitchen describes sending light through a system of detachable connections concluding with a device to measure light loss. If any of those connections were to be broken, the end light detector would receive less light and thus trigger an X alarm.
A more advanced optical fiber based intrusion detection system also implements optical fiber as the sensing medium, but instead of measuring for lost light it analyzes the backscattered light to determine the cause. More specifically, U.S. Pat. No. 5,194,847 to Taylor which is hereby incorporated by reference describes using an interferometer to analyze the patterns of light that are reflected as they are transmitted down an optical fiber. However, Taylor teaches burying optical fiber underground and measuring disturbances based on acoustic or pressure disturbances. Taylor's system is ill suited for intrusion sensing applications.
U.S. Pat. No. 5,705,984 to Wilson describes an intrusion detection system that is based on RF energy as opposed to light. Wilson also buries the cable underground and tests for RF changes caused by deformations in the cable. These deformations are attributable to the weight of an intruder on the cable.
Other inventions implement mechanical devices that are designed to convert a mechanical force into an attenuation of light intensity. Such devices exist but they rely on more than a force on the optical fiber cable itself. In the case of U.S. Pat. No. 4,829,286 to Zvi, a taut wire system is used to trigger a device that attenuates a separate optical fiber. In the case of U.S. Pat. No. 4,777,476 to Dank, a system of hollow tubes and disks translates a force on a fence post into an attenuation of light intensity. Again the direct force is applied to an external medium, the hollow fence post, and not the optical fiber cable. Further, U.S. Pat. No. 5,757,988 to Lindow a system is described that converts the presence of a liquid into an attenuation of light intensity through the optical fiber cable. Again an unrelated stimulus is used to cause a mechanical device to induce attenuation.
Some inventions that rely on breaking connections in the optical fiber cable include U.S. Pat. No. 6,002,501 to Smith that uses optical time domain reflectometer technology to determine an intrusion into barrels of hazardous waste. U.S. Pat. No. 5,055,827 to Phillip describes using optical time domain reflectometer technology to monitor equipment theft. Finally, U.S. Pat. No. 6,265,880 to Born uses optical time domain reflectometer technology to determine the location of chafing of a conduit.